Combined effects of silica fume, aggregate type, and size on postpeak response of concrete in bending

The influences of silica fume, type, and size of aggregate bn the prepeak a nd postpeak response of high-strength concretes in bending were investigate d by measuring the fracture energy G(F), the characteristic length 1(ch), a nd brittleness index B. Degradation of stiffness and strength were also mea sured and a unique focal point was determined using unloading-reloading cyc les during the tests. The degradation of stiffness was correlated to the lo cal fracture energy, strength degradation, permanent crack mouth opening di splacement (CMOD), and permanent displacement at midspan (delta). It was sh own that relations between normalized stiffness, load local energy, CMOD, a nd delta were independent of the partial replacement of cement by silica fu me and of the type and size of aggregate Based on the fracture tests and mi croscopic studies at the matrix-aggregate interface, it was concluded that in both limestone and gravel concretes without silica fume, the cement-aggr egate interface had a large amount of calcium hydroxide and also much less dense calcium silicate hydrate; however, in concretes with silica fume, the interfacial zone became stronger more homogeneous, and dense. In the latte r concretes, the fracture energy decreased dramatically especially when the y contained 20-mm maximum size aggregate, and in these concretes, the britt leness index was substantially high. In gravel aggregate concretes with and without silica fume, cracks developed around the aggregates and generally did not traverse them, due to the particle shape and smooth surface; howeve r, in concretes with silica fume, crack surfaces were less tortuous and fra cture was in a more brittle manner: In limestone concretes with silica fume , the cracks usually traversed the aggregates and a transgranular type of f racture was observed.